gh-37309: README.md: Move all mentions of release tarballs to the installation guide
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As proposed in https://groups.google.com/g/sage-
devel/c/5kmxaw105lg/m/liCPKG4bAQAJ
The README.md now starts with instructions to install git and then use
`git clone`.
- Also fixes https://github.com/sagemath/sage/issues/30201
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URL: https://github.com/sagemath/sage/pull/37309
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik, John H. Palmieri, Matthias Köppe
